1. Field of the Invention
The present invention relates to an isolator/circulator constituting a radio-frequency non-reciprocal element, or in particular to a non-reciprocal element with three central conductors having a small insertion loss and a wide bandwidth characteristic.
2. Description of the Related Art
In the current technical situation of the isolator constituting a radio-frequency non-reciprocal element, the configuration is generally used in which one of the terminals of a three-terminal pair coupled circulator is terminated with a matched impedance. This coupled circulator is classified into two types, i.e. the distributed-element circulator and the lumped-element circulator. The circulator has non-reciprocal electrical characteristics and has such a basic structure that a magnetic field is applied in the direction perpendicular to a ferrite thin plate and a conductor is arranged in proximity to the periphery of the ferrite thin plate. The distributed-element circulator is used in the case where the size of the isolator element is not less than one fourth of the wavelength of the radio frequency propagating through the ferrite thin plate, and the lumped-element circulator in the case where the size of the isolator element is not more than one eighth of the wavelength of the radio frequency. The lumped-element circulator is more suitable for a compact structure.
FIG. 21 is a schematic diagram showing the structure and the circuits of an isolator implemented by connecting a matched impedance (resistor R) to an end of a lumped-element circulator with three terminal pairs currently used with a mobile phone. A ferrite thin plate G is composed of garnet ferrite, and three central conductors L1, L2, L3 are arranged on the upper surface of the ferrite thin plate G at intervals of 120 degrees as shown in FIG. 20. An end of each central conductor makes up an input/output line of the corresponding one of terminal pairs (1), (2), (3), and the other end of the central conductor is connected to a common portion GR constituting a ground conductor. Matching capacitors C1, C2, C3 are connected in parallel between an end of the central conductors L1, L2, L3, respectively, and the common portion GR. The resistor R for absorbing the energy to implement the isolator is mounted between the terminal pair (3) and the common portion GR. A permanent magnet is mounted, though not shown, in such a manner that a static magnetic field is applied in the direction substantially perpendicular to the main surface of the ferrite thin plate G. By carefully adjusting the direction and strength of the static magnetic field and the size of the central conductors L1, L2, L3 and the matching capacitors C1, C2, C3, the structure shown in FIG. 21 operates as a circulator at the desired frequency (hereinafter referred to as “the center frequency”) fo. Thus, the radio frequency input from the terminal pair (1) is propagated to the terminal pair (2), and the radio frequency input from the terminal pair (2) is propagated to the terminal pair (3), with a small loss. As long as the resistor R is connected to the terminal pair (3), most energy is absorbed there, and substantially no radio frequency is propagated to the terminal pair (1) from the terminal pair (2). In other words, an isolator can be implemented in which the propagation is promoted only in one direction and blocked in the other direction.
In the prior art, the intersection angle of the terminal pairs (1), (2), (3) is set normally to 120 degrees. Nevertheless, a non-reciprocal element with three central conductors in which the intersection angle is set to unequal angles has also been proposed. U.S. Pat. Nos. 5,745,014 and 5,994,974 are examples.
The conventional structure shown in FIG. 21, which is symmetric and advantageously easy to fabricate, has the disadvantage is that the insertion loss is not reduced much and the bandwidth is small. Also, the structure disclosed in U.S. Pat. Nos. 5,745,014 and 5,994,974 has the insertion loss and bandwidth not sufficiently improved. These conventional techniques pose the problem that the cost is difficult to reduce.